It is well known in the gas turbine industry that engine performance is proportional to the leakage of working medium gases between opposing seal elements of the rotor and stator assemblies. Techniques and concepts for reducing such clearances are continually under investigation and development.
One class of techniques are those relating to "active clearance control" in which the clearances are set as a function of engine operating condition. The objective is to establish minimum clearances under stable operating conditions, yet to provide sufficient clearance during transient operation to preclude destructive interference between relatively rotating components.
U.S. Pat. Nos. 3,039,737 to Kolthoff entitled "Device for Controlling Clearance Between Rotor and Shroud of a Turbine"; 3,966,354 to Patterson entitled "Thermal Actuated Valve for Clearance Control"; 3,975,901 to Hollinger et al entitled "Device for Regulating Turbine Blade Tip Clearance"; and 4,213,296 to Schwarz entitled "Seal Clearance Control System for a Gas Turbine" are representative of concepts and structures for effecting local control over rotor blade tip clearances. In some embodiments relatively hot air is utilized to move the seals away from the rotor blade tips and in other embodiments relatively cool air is utilized to move the seals toward the rotor blade tips. The concepts are at times combined in the same structure.
Recent commercial aircraft gas turbine engines, such as the JT9D-7R4 engine manufactured by Pratt & Whitney Aircraft, Division of United Technologies Corporation, have incorporated clearance control systems operative on a large segment of the engine to closely match thermal growth of the stator elements to that of the rotor elements. Principally, cooling or heating air is squirted onto the exterior of the engine case of the segment to be controlled. Desired contraction or expansion occurs. U.S. Pat. Nos. 4,069,662 to Redinger et al entitled "Clearance Control for Gas Turbine Engine"; 4,019,320 to Redinger et al entitled "External Gas Turbine Engine Cooling for Clearance Control"; and 4,279,123 to Griffin et al entitled "External Gas Turbine Engine Cooling for Clearance Control" are representative of the concepts employed in systems of the external type.
Advancing techniques for effecting segment cooling now include the wide distribution of cooling air at the interior of the case. Cooling air is flowed along the interior of the engine between the working medium flow path and the engine case.
U.S. Pat. Nos. 3,957,391 to Vollinger entitled "Turbine Cooling"; 3,975,112 to Brown et al entitled "Apparatus for Sealing Gas Turbine Flow Path"; 4,005,946 to Brown et al entitled "Method and Apparatus for Controlling Stator Thermal Growth"; and 4,242,042 to Schwarz entitled "Temperature Control of Engine Case for Clearance Control" representatively illustrated such concepts.
Notwithstanding the effectiveness of such prior art systems, scientists and engineers in the gas turbine engine industry are seeking yet improved systems employing judicious use of cooling/heating air.